Is the cellphone in your pocket public or private?

Cell site simulators, secret surveillance gear that tricks cellphones into transmitting their identifying information and location, have become a preferred method for law enforcement to track people’s whereabouts. Better known as stingrays, the devices mimic legitimate cell towers and induce cellphones in the area to transmit data to the government without ever alerting users. Even when police are looking for a particular suspect, the technology captures information about dozens, hundreds or even thousands of bystanders’ phones. Walls offer no protection, as the stingray’s signals pierce through the walls of homes and other private spaces, revealing otherwise private details about those inside.

Stingrays are typically mounted on police cars or carried by hand, but last year The Wall Street Journal revealed a Justice Department program allowing the U.S. Marshals Service to mount cell site simulators on airplanes operated out of five airports, “with a flying range covering most of the U.S. population.” By flying over major U.S. cities with fake cell towers, the government has been capturing information about tens or hundreds of thousands of people’s phones and interfering with an untold number of calls.

And the DOJ is not alone. For example, in 2010, U.S. Immigration and Customs Enforcement (ICE) bought hardware for installing stingrays in airplanes and paid for several of its agents to be trained in how to operate the equipment. It is unclear where or how often ICE has been flying stingray surveillance operations, but even if the missions were limited to the 100-mile border zone, they would have access to nearly two-thirds of the U.S. population.

Understandably, the news that the feds have been using this invasive surveillance gear on such a large scale caused a stir. In November and December, in four separate letters, more than a dozen U.S. senators wrote to Attorney General Eric Holder and Secretary of Homeland Security Jeh Johnson expressing concern and demanding answers about how often the equipment has been used and what limitations are placed on its use. Some lawmakers have been meeting directly with the Federal Bureau of Investigation to express concerns. In their joint letter sent last month, Sens. Patrick Leahy, D-Vt., and Charles Grassley, R-Iowa, revealed that the FBI has adopted a new policy stating that its agents should get search warrants before using stingrays. Troublingly, however, the senators explained that the policy includes several potentially large loopholes, including uses without a warrant in cases when “the technology is used in public places or other locations at which the FBI deems there is no reasonable expectation of privacy.”

What the FBI means by this exception is unclear. Even when used in a public place, stingrays inevitably send electronic signals through the walls of private homes and offices and into people’s bags and pockets, searching for cellphones within. Unless a stingray is set up in the middle of an uninhabited expanse of wilderness, it will unavoidably search private places. Hence, the public-places loophole must be given a narrower definition.

The U.S. Constitution puts clear limits on sending electronic signals through the walls of homes to gather information from inside. The Supreme Court has ruled that police need a warrant to use infrared cameras or radio beeper transmitters to look for contraband inside houses. The law is clear: Except in narrow emergency circumstances, law enforcement should always get a warrant. The government should not eviscerate the rule by playing fast and loose with the requirements of the Fourth Amendment.

As problematic as federal law enforcement’s use of stingrays is, the bigger threat to privacy is probably the proliferation of the technology among state and local police departments. The American Civil Liberties Union has identified 47 state and local law enforcement agencies in 19 states and the District of Columbia that own cell site simulators. But the actual number of police departments using the technology is likely much higher, because purchases are shrouded in secrecy in deference to nondisclosure agreements with equipment vendors and because many departments borrow the equipment from federal and state agencies. While the FBI is paying lip service to the need for probable cause warrants, many local police departments are using the devices without ever getting warrants.

But there have been some promising developments. Local judges are pushing back against unlawful requests to use stingrays. In Charlotte, North Carolina, judges granted hundreds of police applications to use stingrays on the basis of a low relevance standard rather than the higher probable cause standard required for warrants. The government’s applications lacked any description of stingrays, and because the applications were submitted in secret, judges never heard arguments why warrants and other limitations should be required. Once the local press began raising questions, judges pledged to give greater scrutiny to police requests to use stingrays.

In Tacoma, Washington, local judges learned from a newspaper report that they had signed more than 170 orders (not warrants) over five years that police used to justify stingray surveillance, without police ever mentioning to the judges that those orders were for stingray use. Dismayed by the government’s lack of candor, the judges instituted a requirement that police disclose when they plan to use stingrays and provide an explanation of how they will protect the privacy of nonsuspects.

And in Baltimore, efforts by the police to keep their stingray use secret included a bald refusal to answer judges’ questions in court, citing vague homeland security concerns and nondisclosure agreements with the FBI. In one case, the judge threw out cellphone evidence after police refused to answer his questions about how they located a phone. In another, the judge threatened to hold the testifying police officer in contempt of court. The government opted to drop the cellphone evidence rather than answer questions about its stingray use.

The growing use of stingrays means we need more transparency and controls. In Baltimore, police acknowledged using stingrays more than 900 times from 2007 to 2013, and other departments use the equipment with similar frequency. Without robust oversight from legislators and judges, these searches will continue to sweep in information about countless Americans.

At a minimum, police should be getting warrants and courts, and lawmakers should be demanding privacy protections for us all.